CS Unix Tools & Scripting

Transcription

1 Cornell University, Spring February 24, Slides evolved from previous versions by Hussam Abu-Libdeh and David Slater

2 A note on awk for (item in array) The order in which items are returned is arbitrary.

3 Recap: Passing arguments to scripts When we pass arguments to a bash script, we can access them in a very simple way: $1, $2,... $10, $11 : are the values of the first, second etc arguments $0 : The name of the script $# : The number of arguments $* : All the arguments, $* expands to $1 $2... $n, $@ : All the arguments, $@ expands to $1 $2... $n You almost always want to use $@ $? : Exit code of the last program executed $$ : current process id.

4 You have the power! We now have a variety of UNIX utilities at our disposal and it is time to learn about scripting!

5 Scripting 101 Definition: A script is very similar to a program, although it is usually much simpler to write and it is executed from source code (or byte code) via an interpreter. Shell scripts are scripts designed to run within a command shell like bash. Scripts are written in a scripting language, like perl, ruby, python, sed or awk. They are then run using an interpreter. In our case, the scripting language and the interpreter are both bash.

6 If conditionals If statements are structured just as you would expect: if cmd1 then cmd2 cmd3 elif cmd4 then else fi cmd5 cmd6 Each conditional statement evaluates as true if the cmd executes successfully (returns an exit code of 0)

7 Putting it on one line Sometimes we might want to type a multiline command into the shell, we can do this by hitting enter for each line, or by using semicolons to tell the shell to start new lines: Example: if [ testexpr ] ; then command1 ; command2 ; fi

8 Exercise Let s write a script to send us our weekly tasks (hw2, problem 2), which doesn t send us a blank on weekends.

9 A simple script textsearch.sh #! /bin/bash # This script searches a file for some text then # tells the user if it is found or not. # If it is not found, the text is appended if grep "$1" $2 > /dev/null then else fi echo "$1 found in file $2" echo "$1 not found in file $2, appending." echo $1 >> $2

10 test expressions We would not get very far if all we could do was test with exit codes. Fortunately bash has a special set of commands of the form [ testexp ] that perform the test testexp. First to compare two numbers: n1 -eq n2 : tests if n1 = n2 n1 -ne n2 : tests if n1 n2 n1 -lt n2 : tests if n1 < n2 n1 -le n2 : tests if n1 n2 n1 -gt n2 : tests if n1 > n2 n1 -ge n2 : tests if n1 n2 If either n1 or n2 is not a number, the test fails.

11 Test Expressions We can use test expressions in two ways: test EXPRESSION [ EXPRESSION ] Either of these commands returns an exit status of 0 if the condition is true, or 1 if it is false. Use man test to learn more about testing expressions Note: Remember you can check the exit status of the last program using the $? variable.

12 Example #! /bin/bash # Searches a file for two strings and prints which #is more frequent # Usage:./ifeq.sh <file> string1 string2 arg= grep $2 $1 wc -l arg2= grep $3 $1 wc -l if [ $arg -lt $arg2 ] then echo "$3 is more frequent" elif [ $arg -eq $arg2 ] then else fi echo "Equally frequent" echo "$2 is more frequent"

13 string comparison To perform tests on strings use s1 == s2 : s1 and s2 are identical s1!= s2 : s1 and s2 are different s1 : s1 is not the null string Make sure you leave spaces! s1==s2 will fail!

14 Expansion When using testexp variable substitution is performed, but no matching is perform. If x is the null string, what will [ $x!= monster ] return?

15 Expansion When using testexp variable substitution is performed, but no matching is perform. If x is the null string, what will [ $x!= monster ] return? It will return an error, because $x is expanded to the null string and the test becomes [!= monster ]. To make sure there are no errors, place your variables inside double quotes. Then [ $x!= monster ] is expanded to [ ""!= monster ] which returns true.

16 path testing If path is a string indicating a path, we can test if it is a valid path, the type of file it represents and the type of permissions associated with it: -e path : tests if path exists -f path : tests if path is a file -d path : tests if path is a directory -r path : tests if you have permission to read the file -w path : tests if you have write permission -x path : tests if you have execute permission

17 A long time ago in a galaxy far, far away... we had hw1! Let s understand how it works!

18 More on testing You can combine tests: if [ testexp1 -a testexp2 ] then fi cmd -a : and -o : or! testexp1 : not

19 A note about debugging To debug your code, invoke the script with the -x option. You will then see all the commands successfully executed: $ bash -x ifeq.sh Frankenstein.txt monster the ++ grep monster Frankenstein.txt ++ wc -l + arg=33 ++ grep the Frankenstein.xt ++ wc -l + arg2= [ 33 -lt 3850 ] + echo the is more frequent

20 Testing arguments We can now begin to ensure our scripts get the input we want: if [ -f $1 ] then else fi Perform the action you want echo "This script needs a file as its input dummy!"

21 Arithmetic A little arithmetic can be useful and BASH can perform all the standard operators Arithmetic a++, a : Post-increment/decrement ++a, a : Pre-increment/decrement a+b, a-b : Addition/subtraction a*b, a/b : Multiplication/division a%b : Modulu a**b : Exponential a>b, a<b : Greater than, less than a==b, a!=b : Equality/inequality =, +=, -= : Assignments

22 Using Arithmetic Expressions We have already seen one way to do arithmetic: Example: echo $((2+5)) 7 We can also use it as part of a larger command: The Let Built-In VAR1=2 let VAR2=$VAR1+15 let VAR2++ echo $VAR2 18 let evaluates all expressions following the equal sign

23 The Difference all characters between the (( and )) are treated as quoted (no shell expansion) The let statement requires there be no spaces anywhere (so need to quote) Both work only with integers, for real numbers use bc. Example: let "i=i + 1" i=$(($i + 1))